#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
#include<cmath>
using namespace std;
int main()
{
	int n;
	cin >> n;
	double sum = 0;
	for (int i = 1; i <= n; i++) {
		double a = sqrt((double)i);
		sum += a;
	}
	printf("%.2lf", sum);
	return 0;
}